1. Field of the Invention
The present invention relates to a device and method for enhancing output current driving, and more particularly to a device and method for enhancing current driving by using an auxiliary output device to output an auxiliary current.
2. Description of the Related Art
A conventional operational amplifier can be used for amplifying a signal difference between two input signals. Generally, an output voltage of a voltage operational amplifier is equal to a difference between a non-inverse input voltage and an inverse input voltage multiplying a voltage gain of the operational amplifier. An ideal voltage operational amplifier has high input impedance, and its output impedance is near 0. Regarding an operational transconductance amplifier (OTA), its output current is equal to a difference between its non-inverse input voltage and inverse input voltage multiplying a transconductance gain of the operational amplifier. An ideal OTA has high input impedance and a high output impedance.
For a conventional operational amplifier, in order to achieve rail-to-rail effect, when a range of an output voltage is proximately close to a range of an input voltage, and to obtain great current driving, an output stage usually is added after the operational amplifier.
FIG. 1 is a schematic drawing showing a conventional operational amplifier. Referring to FIG. 1, the operational amplifier 100 comprises an OTA 102 and an output stage 104. The non-inverse input voltage of the OTA 102 is V1, and its inverse input voltage is V2. The output stage 104 comprises a pull-push output stage, which is composed of two metal-oxide-semiconductor (MOS) transistors 112 and 114. The output terminal of the OTA 102 is coupled to the input terminal of the output stage 104. The output voltage of the output stage 104 is VO, and coupled to the load 106. The voltage range of the input source of the operational amplifier 102 is between the VDD and ground, 0V. The source of the PMOS transistor 112 of the output stage 104 is coupled to the voltage VDD, and the source of the NMOS transistor 114 is grounded. Due to its high output impedance, the OTA 102 is not adapted for driving the load 106 with low resistance or high capacitance. By using the push-pull output stage 104 to push or pull a current from the load 106, the current driving and the output voltage range are enhanced.
In some applications, the conventional operational amplifier used in a unit gain buffer, however, generates great output offset.
FIG. 2 is a schematic drawing showing an application of a conventional operational amplifier circuit. Referring to FIG. 2, the output terminal of the output stage 104 is coupled to the inverse input terminal of the OTA 102. Generally, this connection method is called negative feedback. For example, if the voltage VO is close to the maximum voltage VDD of the input source, while outputting a great push current to the load 106, a great output offset is created. Traditionally, a method of reducing the output offset is to lower the impedance of the output terminal of the output stage 104 to the positive voltage source VDD, that is, to increase the aspect ratio of the PMOS transistor 112. In addition, if the voltage VO is very close to the minimum voltage 0V of the input power source, while pulling a great pull current from the load 106, a great output offset also is created. A traditional method of reducing the output offset is to lower the impedance of the output stage 104 to the negative voltage source, i.e., the ground terminal, that is, to increase the aspect ratio of the NMOS transistor 114.
Accordingly, if the conventional operational amplifier includes both the rail-to-rail function and great current driving, the aspect ratios of the transistors 112 and 114 of the output stage 104 should be increased. The conventional circuit, however, increases the circuit layout area and also the cost. In addition, it also enhances the steady current flowing from the transistor 112 to the transistor 114 of the output stage 104, causing power loss. Accordingly, an operational amplifier which can achieve the rail-to-rail output, have a great current driving, and reduce the aspect ratio of the transistor and cost without increasing the steady current is desired.